feat: clawsync diff command + revision_page_entries API + CHANGELOG

diff command:
  clawsync diff <file.h5> <rev1> [rev2]
  - compares page-level metadata between two revisions (or rev1 vs HEAD)
  - ~modified / +added / -removed output per page
  - --porcelain: tab-separated machine-readable output
  - detection via data_offset equality (append-only sidecar invariant)

clawhdf5-onion:
  OnionFile::revision_page_entries(rev) -> &[PageTableEntry]
  - zero-copy access to page table entries for diff and analytics

4 subprocess tests: identical/changed/head-default/porcelain

CHANGELOG.md: Keep-a-Changelog v0.1.0 entry covering all 8 crates,
all CLI commands, and benchmark numbers.

Co-Authored-By: Claude Sonnet 4.6 <[email protected]>

crates/clawhdf5-onion/src/reader.rs

@@ -199,6 +199,22 @@ impl OnionFile {
     /// callers that intend to send the data over the network can ship the
     /// compressed bytes directly and let the receiver decompress.
     ///
+    /// Returns the raw [`PageTableEntry`] slice for a revision.
+    ///
+    /// Useful for comparing page metadata across revisions without loading the
+    /// actual page data.  The `data_offset` field uniquely identifies the
+    /// stored content: two entries with the same `data_offset` contain identical
+    /// (compressed) bytes.
+    pub fn revision_page_entries(
+        &self,
+        rev: u64,
+    ) -> Result<&[crate::format::PageTableEntry], OnionError> {
+        self.page_tables
+            .get(rev as usize)
+            .map(|v| v.as_slice())
+            .ok_or(OnionError::RevisionNotFound(rev))
+    }
+
     /// Returns `(h5_offset, compressed_data, codec_byte, orig_size)`.
     pub fn revision_pages_raw(&self, rev: u64) -> Result<Vec<RawPage>, OnionError> {
         let table = self